I've been learning Ancient Greek for the past year as part of my course at university, using Athenaze books one and two (supplemented in the parts that they lack by my wonderful tutor!), but I'm planning on taking either some Latin modules or Ancient History modules instead of continuing onto Intermediate Greek next year to try and keep my breadth of study as wide as possible - when Greek takes 2 of your 3 module credits, learning Classics can become secondary to the language.

I'd still like to continue my studies in Greek, though. We have completed Athenaze up to book two, unit 19 - everything up to the future tense. Does anyone have any suggestions for additional texts I could study? I'm planning on completing the rest of Athenaze, although I know my tutor doesn't use them in learning at intermediate level.

What have you covered so far James? I'm afraid I don't know the Athenaze course, but if you've nearly finished this two book course you're probably ready to start translating some original text. It was a big shock when we first started translating Lysias, it's totally different from just translating the odd sentance in an excercise book. After a while you get used to it though.
I didn't really like Lysias, but that's probably because I found it really hard to understand (I failed my exam that year in ancient Greek). Next we translated Plato (the Apologia), which I understood much better (lol, top marks in that exam ). I also found Plato very interesting, but I suppose Lysias would have been more fun if it hadn't been for the fact that I was unable to translate any of it. Lysias was a good start I suppose because his speeches give you a glimps of what life in Athens was like for the everyday man (Lysias wrote many defences speeches, as in Athens at that time everyone had to defend themselves in the courts).
Plato will probably be more interesting for you though, as you're studying classics and he's such an important philosopher - so I'd recommend starting with Plato .

- That's pretty much it, though I'm sure there are things I've missed. We've been translating little snippets from the original for quite a while, using Nairn & Nairn's Reading Ancient Greek book. There have also been bits of Herodotus thrown in for good measure.

Hmm... seems like the future is the most important thing missing, which I find surprising as it was the second tense we learned after present, as it's very easy (just stick a sigma in the ending ). The only thing is that of course its passive forms (just like those of the Aorist) are different from those of the middle case.
So if you can I'd just take a look at the future and plusquam perfect (that's easy too, but not as important as future).

I think you could start translating a text, as I said Lysias or Plato for example, but really what ever you fancy. I would suggest you buy a small grammar, if you don't have one already.
I have a very concise Greek grammar (and a really large one), but I find the concise one very good for reference. When you're translating an original text first time and your main goal is to translate and understand the text, then you don't really want to have to go through a huge book just to find out what the various uses of the optative or conjunctive in Greek are or how to translate many negations or even if you just want to look at the various tenses again to make sure you've analysed a form correctly. Uhh.. and of course you'll need some dictionary.
If you aren't too sure whether you really want to pay out for an expensive dictionary, then search around for readers, which have all the vocab you'll need for that text at the back of the book.